108 Introduction to the Study of Science 



assumes as pervading the whole universe of space and matter. 

 This medium is called the ether. The ether envelops and 

 permeates all things; it fills the infinite space between the' 

 heavenly bodies of the universe and the most minute spaces 

 between the molecules of all solids, liquids, and gases. It is 

 supposed to be rigid and at the same time elastic, offering no 

 resistance to bodies passing through it. Because of these 

 properties it transmits radiant energy in all directions and in 

 straight lines by radiation or in the form of waves. It is the 

 medium of transfer of radiant energy. Through this medium 

 the sun's energy passes with a velocity of about 187,000 miles 

 a second, but the medium itself is not heated. 



Effects of radiant energy. The molecular vibrations of a 

 body, such as of the sun or a hot range, are communicated to 

 the all-pervading ether, causing it to vibrate. When the 

 vibrations radiating in all directions along straight paths fall 

 upon a body, they promote or accelerate its molecular activity, 

 that is, they heat the body. As the body's molecular vibrations 

 become more rapid and violent, its molecules are farther and 

 farther repelled by and from one another ; this appears to us 

 as the expansion of the body. Air, as you know, -expands 

 with rising temperature. All other gases, liquids, and solids 

 behave in a similar way. 



When a body, on the contrary, is no longer exposed to the 

 vibrations from a source of radiant energy, as the sun, it gives 

 up or radiates its own energy more rapidly than it receives 

 energy. Its molecular vibrations diminish, the paths of the 

 molecules become slower and shorter, or, as we commonly 

 express it, the body contracts. Contraction with the lower- 

 ing of temperature is characteristic of all substances except 

 water. 



Heat is only one result of radiant energy. Radiant energy 

 of the sun or any other source, such as a lamp, has different 

 effects as determined by the nature of the body or the organ 

 upon which it falls. The sun's energy falling upon the earth 



